The present invention relates to ink jet printing apparatuses for use with computer systems or the like and, more particularly, to an on-demand type ink jet printer in which ink droplets are formed and caused to fly in response to electric signals to reproduce on a sheet of paper characters, numerals, symbols and other data as combinations or patterns of resulting dots.
Various ink jet printing systems have heretofore been proposed and put to practical use by virtue of their merit that information can be directly reproduced on ordinary sheets of paper without any impact. In the on-demand type system, an ink droplet in formed by an electric signal and caused to fly to impinge on a paper sheet so that the resulting dot prints out desired information in combination with other dots. Such a system renders an ink jet printer compact design because it does not require deflection of flying ink droplets and, accordingly, a deflection device. The electric signal for forming ink droplets can be suitably modulated to vary the amount of ink of an ink droplet and, thereby, the size of the dot, enabling the tone of reproduced images to be controlled within a certain range.
The on-demand ink jet printer is required to be capable of multi-port ink ejection with a plurality of ink ejection heads which correspond to a desired number of dots. Another requirement is that an electric field for shaping ink droplets be provided with a sufficient intensity in order to accomplish efficient formation and stable atomization of ink droplets.
To satisfy these requirements, an ink jet printer may be provided with an ink ejection nozzle and develop an electric field at a leading end portion of the nozzle, as disclosed in Japanese Utility Model Publication No. 54-19874/1979. Though this kind of ink jet printer is successful in achieving a sufficient intensity of electric field for the effective formation of ink droplets, it is difficult to provide the ink ejection head with a multi-port design. Alternatively, an ink jet printer may employ a plurality of control electrodes disposed in a single slot for ink ejection, as described in Japanese Patent Laid-Open Publication No. 56-170/1981 or 56-42664/1981. While such an alternative design promotes a multi-port ejection head construction due to the fact that ink droplets are formed by each of the control electrodes, it still fails to attain a sufficient intensity of electric field required for forming ink droplets. Additionally, it makes the atomization of ink into droplets unstable depending on the configuration of the slot.